1. Technical Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, especially to a semiconductor device in which resistor elements are integrated on a semiconductor substrate.
2. Background of the Invention
Resistor elements have been used in a variety of semiconductor integrated circuits, including resistors for delay circuits, resistors for oscillator circuits, and ladder resistors for Analog-Digital converters. FIG. 11 is a cross-sectional view of the structure of a prior art semiconductor device. On N− type semiconductor substrate 50, field oxide films 51 are formed. P− type resistance layer 52 is formed on the surface of the N− type semiconductor substrate 50 between the field oxide films 51. Also, on both sides of the P− type resistance layer 52, P+ type electrode pad layers 53 and 54 are formed.
FIG. 12 is shows a cross section of the semiconductor device of FIG. 11 in use. The voltage VL is applied to the electrode pad layer 53, and the voltage VH is applied to the electrode pad layer 54 in the Figure. Hence, when the voltage of the N type semiconductor substrate 50 is 0V, it is supposed that VH<VL<0V. That is, forward bias of the P+ type electrode pad layers 53 and 54 and the N type semiconductor substrate 50 is prevented. Also, in terms of absolute value, voltage VH is greater than voltage VL. Therefore, electric current goes through the P− type resistance layer 52 according to the voltage difference (VH−VL).
When the resistance layer 52 is used as a resistor element in a semiconductor integrated circuit, it is desirable that there be no voltage dependence of the resistance value for the sake of circuit design. However, when voltage VH is applied to the P+ type electrode pad layer 54, the depletion layer 55 between the N type semiconductor substrate 50 and the resistance layer 52 is expanded. Therefore, the P− type resistance layer 52 is narrowed down, causing the change in a resistance value, which depends on the voltage VH applied to the P+ type electrode pad layer 54. Also, when the voltage VH rises further, a pinch-off state takes place near the P+ type electrode pad layer 54, leading to saturation of the electric current.